A Synthesis of the Structural Geogoly of the Northern Transvaal

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A SYNTHESIS OF THE STRUCTURAL GEOLOGY OF THE NORTHERN TRANSVAAL by CARLOS JORGE CHERNICOFF Dissertation submitted in fulfilment of the requirements for the degree of MASTER OF SCIENCE in GEOLOGY FACULTY OF SCIENCE at the RAND AFRIKAANS UNIVERSITY Supervisor Prof. C. Roering Co-supervisor Prof. D.O. Van Reenen NOVEMBER 1984 - SUt·1MARY - The study area is subdivided into seven tectonic domains, viz. Bandelierkop, Southern Beit Bridge Complex, Alldays, Hestern Transvaal, Soutpansberg, Waterberg and Lebombo domains. The Bandelierkop domain comprises the Southern Marginal Zone of the Limpopo Belt (Bdl) and the northern portion of the Kaapvaal craton (Bd2). Bdl is characterized by the presence of highly disrupted xenoliths of pelitic and mafic supracrustal rocks distributed in a "sea " of granitic ·material. This contrasts with the much bigger supracrustal xenoliths in Bd2, i.e. the greenstone belt relicts. The granulite facies rocks of Bdl may have been upthrust roughly from south to north along one or more south-dip ping thrust faults soling into a gently-dipping to flat lying basal shear zone. Since this model reveals the e xistence of the Kaapvaal craton partly on edge, the pro gressively higher grade of regional metamorphism (from greenschist to granulite facies) encountered in the Bande lierkop domain, from south (Bd2) to north (Bdl), could express the transition from upper crust to lower crust as exposed on the present land surface. The structural trends found in Bdl and Bd2 are not strictly confined to either area, and some overlapping exists. The most distinct structural trend in Bdl is a west-north west fold trend, a northeast fold trend also being recog nized in the eastern portion of Bdl. Bd2 exhibits f~ld trends varying from west-northwest to northeast in orienta tion; in this area there is ample evidence for the later nature of the northwest- to west-northwest oriented folding event. The east-northeast- to northeast folding is con ~dered to represent an event independent of the effect of deformation of the Kudus River shear zone to which this folding was previously thought to be related. The portion of the Central Zone of the Limpopo Belt falling within South African territory comprises the Alldays and Southern Beit Br_idge Compl ex (SBBCd) doma tn s., There e xists a marked contrast between the roughly north trending fold trends of the Alldays domain and the east-northeast fold structure of the southernmost part of the Central Zone, the SBBCd. The relatively gentle plunges of the fold structure of the Alldays domain may have accentuated the size of the regional folds in this region. The SBBCd occupies a narrow east-northeast oriented strip extending immediately south of the Alldays domain, where tightly folded gneisses and metasedim~nts are exposed. all strata are strongly aligned in the regionally extensive east northeast direction. A contrasting difference between the SBBCd and the surrounding Precambrian terranes is clear, as the former domain conveys a relatively higher strain; this evidence of heterogeneous strain suggests that the SBBCd represents a shear zone. The shearing movement may have been of thrust type, consistent with the thrust model re ferred to above, and the SBBCd may be regarded as the thrust plane. In'iiew of the steeply-dipping, south-southwest oriented fold axes known to exist along part of the SBBCd (area south of Messina), it would appear that, at least in that area, the thrust movement would have been towards the north-northeast. A late, involved history of deformation followed in the SBBCd, during which one or more episodes of wrench-type movement may have taken place. The Soutpansberg domain is a relatively narrow and long fault zone of Proterozoic to Phanerozoic age that separates 'mobile belt' environment to the north, from 'cratonic' environment to the south. A linked fault system character ized by south-dipping, normal listric faults soling into ? gently-dipping major detachment surface may account for the structural pattern of this domain. The thrust system referred to above, which developed earlier in the geological ./ history of the region along the Limpopo Belt - Kaapvaal craton boundary, may have controlled the late extensional displacement in the Soutpansberg domain. The overall structure of the Waterberg domain would seem to be controlled by the superimposition of northeast- and northwest oriented gentle folds and, with the exception of locally intense deformation in the early Waterberg basin, would appear to have developed on a relatively stable portion of crust. The structure of the Lebombo domain is characterized by a north-south striking monocline with gentle east dip. Most of the lineaments inferred from the aeromagnetic sur vey of the study area originate from dyke intrusions. Faulting, fracturing and shearing are less clearly mani fested in the pattern of the aeromagnetic contour maps, instead they are better correlated with the lineaments inferre~ from LANDSAT imagery and from the drainage patterns of the region. Opsomming Die studiegebied word onderverdeel in sewe tektoniese domeine, naamlik die Bandelierkop-, Suidelike Beitbrug kompleks, Alldays-, Westelike Transvaal-, Soutpansberg-, Waterberg- en lebombodomeine. Die Bandelierkopdomein sluit ~·ie suidelike randsone van die Limpopogordel (Bdl) en die noordelike gedeelte van die Kaapvaalkraton (Bd2) in. Bdl word gekenmerk deur die teenwoordigheid van uiters uiteengeskeurde xeno liete van peliti~se en mafiese deklaag gesteentes, d.w.s groensteengordeloorblyfsels, in Bd2. Die granulietfasies gesteentes van Bdl kon opgehef gewees het van suid na noord langs een of meer suidhel1ende listriese stootverskuiwings wat oorgaan in 'n vlakhel lende basale skuifskeursone. Aangesien hierdie model die hellende voorkoms van die Kaapvaalkraton aandui, kan die progressiewe ho~rgraad van regionale metamorfose (van groenskis tot granulietfasies) wat in die Bandelier kopdomein aangetref word die oorgang van die vlakker tot die dieper kors aandui soos blootgele op die teen woordige landoppervlakte. Die struktuurneigings wat in Bdl en Bd2 aangetref word, is nie streng tot die een of die ander gebied beperk nie, maar daar is 'n mate van oorvleueling. Die duidelikste struktuurneiging in Bdl is 'n WNW plooineiging, terwyl 'n NO plooineiging in die oostelike gedeelte van BDl op gemerk word. Bd2 vertoon plooineigings wat wissel van WNW tot NO in ori~ntasie; in hierdie gebied is daar vol doende bewys vir die later aard van die NW tot WNW ge ori~nteerde plooiingsverskynsels. Die aNa tot NO plooi ing word beskou as In verskynsel wat onafhanklik is van die vervormingseffek van die Koedoesrivierskuifskeursone. Die gedeelte van die Sentrale Sone van die Limpopo gordel binne Suid-Afrikaanse gebied sluit die Alldays en die Suidelike Beitbrugkompleksd6meine (SGBCd) in, Daar bestaan In duidelike kontras tussen die noordstrek kende plooineigings van die Alldaysdomein en die ONO plooistrukture van diemees suidelike gedeelte van die Sentrale Sone -- die SBBCd. Die relatiewe vlak duiking van die Alldaysdomein-plooistrukture kan die grootte van die regionale plooie in hierdie gebied beklemtoon. Die SBBCd beslaan In smal ONO-geori~nteerde strook on middelik suid van die A1ldaysdomein waar sterkgep100ide gneise en metasedimente blootgele word; al die strata is sterk gerig in die regionale OND rigting. In Kontras terende verskil tussen die SBBCd en die omliggende voor Kambriese gebiede is ooglopend aangesien die eersgenoem de domein In relatief ho~r mate van vervorming aantoon. Hierdie bewys vir onge1yksoo~tige vervorming wi1 daar- op dui dat die SBBCd In skuifskeursone verteenwoordig. Die sKuifskeurbeweging kon 'n tipe stootversKuiwing ge wees het in ooreenstemming met die stootverskuiwings model waarna hierbo verwys is, en die SBBCd kan as die stootverskuiwingsvlak beskou word. In die 1ig van die stylhellende SSW-geori~nteerde plooi asse, wil dit voorkom asof die stootverskuiwingsrigting in daardie gebied na die NNO was. In laat en ingewikkel de vervormingsgeskiedenis het in die SBBCd gevolg in die loop waarvan een of meer episodes van verwringing kon plaasgevind het. Die Soutpansbergdomein is 'n re1atief smal en lang ver skuiwingsone van die Proteroso~ese tot die Phaneroso~ese Tye wat die mobiele gordel-omgewing in die noorde van die kr a t onies e omgewing in die suide skei. 'n Gekoppelde verskuiwingsisteem wat gekenmerk word deur suidhellende listriese afskuiwings wat in In geleidelik-hellende hoof verp1asingssone oorgaan, kan In verklaring bied vir die struktuurpatroon van hierdie domein. Die stootYer skuiwingsisteem waarna hierboverwys is en wat vroe~r in die geologiese geskiedenis van die gebied langs die Limpopogordel-Kaapvaalkratongrens

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